CFM fan question

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kronic&bars

New Member
alright this is probably a stupid question but i got no idea when it comes to the ventilation. anyway, so my grow area is gonna be 2x4x5 ft. which is 40 square ft. does that mean i should use a fan with 40 CFM?
 
K

krowleey

Member
i have a 2x4x9 and use a 400 cfm with a carbon filter for exhaust. and i also use a 1k light air cooled of course hooked into said exhaust. I use a 6 inch passive air intake so i get great negative pressure.
 
K

krowleey

Member
btw this is my first grow, so far my temp has not gotten over 78. as long as i keep my back bedroom cool, my grow room stays cool. i can put my hand touching the glass of that 1k light it doesnt burn. so i vote you get a 400 cfm fan and run a vented light hood.
 
crunkyeah

crunkyeah

Well-Known Member
@OP, let me enlighten you on something I noticed in your post. Your grow area is 2'L x 4'W x 5'H. Square footage is exactly that, a square. That is a 3D figure, so you're looking for cubed feet not square feet. You're square feet is only 8. Then you multiply by 5 to get your cubed feet. That is the number you have correct, just pointing out that it's cubed feet not square feet.

Here's a link you may find extremely beneficial, here. Just type in your measurements and it will tell you what you need. You're looking at roughly 6 exchanges every hour I believe. The higher number per hour is actually better, but you're looking at an air exchange roughly every 10 minutes. You can get away with 2 exchanges per hour, but I've found the more fresh air plants get the better they do.

Like krowleey pointed out, you want a higher output coming out of your exhaust than you want coming in. This creates backpressure and makes the exhaust fan last longer since the pressure is going with it and not against it. You'll also want exhaust near the top, and intake near the bottom. It's also a good idea to have an oscillating fan in there for circulation/stem strengthening/soil drying. This will help against pests and weak stems.

Hope this helps.
 
K

kronic&bars

New Member
crunkyeah said:
@OP, let me enlighten you on something I noticed in your post. Your grow area is 2'L x 4'W x 5'H. Square footage is exactly that, a square. That is a 3D figure, so you're looking for cubed feet not square feet. You're square feet is only 8. Then you multiply by 5 to get your cubed feet. That is the number you have correct, just pointing out that it's cubed feet not square feet.

Here's a link you may find extremely beneficial, here. Just type in your measurements and it will tell you what you need. You're looking at roughly 6 exchanges every hour I believe. The higher number per hour is actually better, but you're looking at an air exchange roughly every 10 minutes. You can get away with 2 exchanges per hour, but I've found the more fresh air plants get the better they do.

Like krowleey pointed out, you want a higher output coming out of your exhaust than you want coming in. This creates backpressure and makes the exhaust fan last longer since the pressure is going with it and not against it. You'll also want exhaust near the top, and intake near the bottom. It's also a good idea to have an oscillating fan in there for circulation/stem strengthening/soil drying. This will help against pests and weak stems.

Hope this helps.
Click to expand...

ya cubed feet, thats what i meant. thanks for your help, that link is very helpful
 
crunkyeah

crunkyeah

Well-Known Member
Glad I could be of assistance :) I was looking into my grow info and I found that your exhaust should be cubed feet divided by 5. This should be the CFM output of the fan. The intake should be divided by 3. This confirms the backpressure thing as well.

Cheers :)
 
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